Hepatic microsomal glucose-6-phosphatase of normal and alloxan-diabetic rats: Thermotropic effects on kinetics and interaction with deoxycholate and 1-anilino-8-naphthalene sulfonate

Abstract

Thermotropic effects on the kinetics of glucose-6-phosphatase ( d-glucose-6-phosphate phosphohydrolase, EC 3.1.3.9) activity of hepatic microsomes from normal and alloxan-diabetic rat liver were investigated by determining V, K m and K i (substrate inhibition) values. Influence of deoxycholate (0.1%) and 1-anilino-8-naphthalene sulfonate (2.5 mM) on the kinetics was also evaluated. 1. 1.|Substrate inhibition occurred at 0.06 M for the enzyme from normal rats and at 0.02–0.025 M for the enzyme from diabetic rats. 2. 2.|The enzyme from diabetic rats showed a transition that extended between 22.7 and 27°C in the Arrhenius plot (log V vs. T −1) instead of at 19.5°C. 3. 3.|Deoxycholate increased the V value of both enzymes without affecting substrate inhibition at all the temperatures but did not completely abolish the transition in the Arrhenius plot of the enzyme from diabetic rats. 4. 4.|1-Anilino-8-naphthalene sulfonate eliminated substrate inhibition and activated the enzyme of normal rats above 27.5°C by increasing both V and K m values. Below this temperature, the enzyme showed biphasic or allosteric kinetics. At low substrate concentrations the enzyme was inhibited competitively, whereas at high substrate concentrations it was activated as both V and K m values were increased. The enzyme from diabetic rats, on the other hand, was activated at all the temperatures and exhibited linear kinetics. 5. 5.|Binding of 1-anilino-8-naphthalene sulfonate to the microsomal fraction increased with decreasing temperature as revealed by the increase of relative fluorescence. The microsomal fraction of diabetic rats showed a more anomalous fluorescence response between 13–18°C. 6. 6.|Enthalphy changes for glucose 6-phosphate binding to the inhibition site were slightly larger than binding to the active site. Calculated entropies of activation for transition state complex of glucose-6-phosphatase reaction were fairly large and negative. The free energy of activation (28–30 kcal/mol) was independent of temperature and experimental conditions. 7. 7.|In the microsomal fraction (total as well as rought), phospholipid content and fatty acid unsaturation index of phospholipids were decreased after diabetes. The level of free cholesterol remaiined unchanged but the molar ratio of cholesterol to phospholipid increased. The different thermal response and 1-anilino-8-naphthalene sulfonate interaction of the enzyme from diabetic rat liver could be ascribed to the altered lipid environment of the enzyme on the endoplasmic reticulum membrane.